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      <h1 class="post-title">Java的Exception体系</h1>

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        <span class="post-time"> 2022-03-21 </span>
        
          <span class="more-meta"> 约 1112 字 </span>
          <span class="more-meta"> 预计阅读 3 分钟 </span>
        
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  <h2 class="post-toc-title">文章目录</h2>
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    <li><a href="#java异常层级">Java异常层级</a></li>
    <li><a href="#使用检查型和非检查型异常的基本准则">使用检查型和非检查型异常的基本准则</a></li>
    <li><a href="#spring-boot当中是使用runtimeexception还是其他的exception">Spring Boot当中是使用RuntimeException还是其他的Exception</a></li>
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      <p>众所周知，Java异常分为两大类：检查弄(checked)异常和非检查型(unchecked)异常。</p>
<h2 id="java异常层级">Java异常层级</h2>
<p>先来看看Java异常层级图：</p>
<p><img src="/img/Java/Java%E5%BC%82%E5%B8%B8%E5%B1%82%E6%AC%A1%E7%BB%93%E6%9E%84.png" alt=""></p>
<p>所有异常都是由Throwable继承而来，但在在下一层立即分解为两个分支：Error和Exception。</p>
<p>Error类层级结构描述了Java运行时系统的内部错误和资源耗尽错误，例如OutOfMemoryError。你的应用不应该抛出这种类型的对象。如果出现了这样的内部错误，除了通知用户，并尽力妥善终止程序之外，你几乎做了不什么处理。这各情况也非常少出现。</p>
<p>在设计Java程序时，着重关注Exception以及基子类。Exception又分解为两个分支：一个是派生于RuntimeException；另外一个包含其他异常。一般的使用规则是：由编程错误导致的异常属于RuntimeException；如果程序本身没有问题，但由于像I/O错误这类问题导致的异常属于其他异常。</p>
<p>派生于RuntimeException的异常通常包括如下问题：</p>
<ul>
<li>错误的强制类型转换</li>
<li>数组访问越界</li>
<li>访问null指针</li>
</ul>
<p>不派生于RuntimeException的异常包括：</p>
<ul>
<li>访问一个不存在的文件</li>
<li>超载文件末尾继续读取数据</li>
<li>根据给定的字符串查找Class对象，而这个字符串表示的类并不存在</li>
</ul>
<h2 id="使用检查型和非检查型异常的基本准则">使用检查型和非检查型异常的基本准则</h2>
<ul>
<li>如果可以合理的期望客户端（调用端）从异常中恢复，则使用检查型异常</li>
<li>如果客户端（启用端）无法从异常中恢复，则使用非检查型异常</li>
</ul>
<p>举个例子，在打开一个文件之前，我们需要先验证文件名。如果用户输入的文件名是非法的，我们可以抛出一个自定义的检查型异常。</p>
<div class="highlight"><div class="chroma">
<table class="lntable"><tr><td class="lntd">
<pre class="chroma"><code><span class="lnt">1
</span><span class="lnt">2
</span><span class="lnt">3
</span></code></pre></td>
<td class="lntd">
<pre class="chroma"><code class="language-java" data-lang="java"><span class="k">if</span> <span class="o">(!</span><span class="n">isCorrectFileName</span><span class="o">(</span><span class="n">fileName</span><span class="o">))</span> <span class="o">{</span>
    <span class="k">throw</span> <span class="k">new</span> <span class="n">InvalidFilenameCheckedException</span><span class="o">(</span><span class="s">&#34;非法的文件名&#34;</span><span class="o">);</span>
<span class="o">}</span>
</code></pre></td></tr></table>
</div>
</div><p>这样的话，客户端（调用端）在得到异常之后，可以要求用户输入另外一个合法的文件名。</p>
<p>然而，如果客户端（调用端）传入了一个null或者空的字符串，意味着在代码当中出现了某些错误，这各情况可以抛出一个非检查型异常。</p>
<div class="highlight"><div class="chroma">
<table class="lntable"><tr><td class="lntd">
<pre class="chroma"><code><span class="lnt">1
</span><span class="lnt">2
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<td class="lntd">
<pre class="chroma"><code class="language-java" data-lang="java"><span class="k">if</span> <span class="o">(</span><span class="n">fileName</span> <span class="o">==</span> <span class="kc">null</span> <span class="o">||</span> <span class="n">fileName</span><span class="o">.</span><span class="na">isEmpty</span><span class="o">())</span> <span class="o">{</span>
    <span class="k">throw</span> <span class="k">new</span> <span class="n">FilenameUncheckedException</span><span class="o">(</span><span class="s">&#34;未知错误&#34;</span><span class="o">);</span>
<span class="o">}</span>
</code></pre></td></tr></table>
</div>
</div><h2 id="spring-boot当中是使用runtimeexception还是其他的exception">Spring Boot当中是使用RuntimeException还是其他的Exception</h2>
<p>这个问题在我刚开始使用Spring Boot的时候困扰了我很久，现在我有了一个统一的答案，那就是上面的原则。但是再加一条，那就是优先考虑使用检查型(checked)异常，尽可能把问题都暴露出来，这样的话在别人调用代码的时候就知道什么异常可能会发生，然后做出必要的处理。而不是等到了上线之后发现一堆的runtime exception，这样对用户体验是非常差的。</p>

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